The present invention relates to a substrate voltage generator used in a semiconductor memory device, and more particularly to a charge pump circuit for providing the substrate with a negative voltage.
Generally, the substrate voltage generator is used in a semiconductor memory device, to stabilize the threshold voltages of MOS transistors and to prevent the occurrence of parasitic transistors as well as malfunctioning due to the undershoot of source signals. Specifically, it is well-known in this technical field that in a NMOS transistor comprising a p-type substrate and n-type channel if a negative voltage is applied to the substrate, it is possible to prevent occurrence of parasitic transistors and stabilize the threshold voltage.
Referring to FIG. 1 for schematically illustrating the system of a substrate voltage generator, if the substrate voltage (V.sub.BB) 50 is in an undesirable level, the detector 10 detects the level to regulate the oscillator 20. Then signals generated by the oscillator drive a drive circuit 30 of the charge pump circuit 40 to convert the V.sub.BB of a substrate rode 50 to a desired level. The present invention is directed to the charge pump circuit 40.
In a conventional charge pump circuit of the substrate voltage generator as shown in FIG. 2, the charge pump circuit is driven by the signals CLKI to CLK4 so as to discharge current from the substrate ground, thus dropping the substrate voltage. In this case, when the voltage of n22 (n25) is dropped to the minimum -V.sub.cc (V.sub.cc being the source voltage) by means of M22 (M25) driven by the CLK2 (CLK3), the voltage of n21 (n26) is also dropped to only -V.sub.cc by means of M21 (M26) driven by the CLK1 (CLK4), and therefore the V.sub.BB may not be dropped below the minimum -V.sub.cc -V.sub.TP (Here, V.sub.TP is the threshold voltage of M27, usually about 1.0V). Hence this conventional circuit has a low pumping capability with a low V.sub.CC so as not to provide a stabilized substrate voltage.